Shannen Seldon*
April Li
Balnur Zhaisanbayeva
Shamoli Ahmed
Poppy Green
Cepaea nemoralis are considered to be highly polymorphic as they show obvious differences in both colour and number of bands. Polymorphism has long been of evolutionary interest, as directional selection in a population would seem to lead to the existence of the most advantageous type. C. nemoralis are also small and easy to collect, and due to their lack of mobility we can observe multiple generations at once, thus making it the ideal organism to study.
Biologists have found the population structure of snails quite conspicuous. They varied in size with little migration between populations, also amongst populations the distribution varied. In some populations banded snails dominated, whereas in others unbanded snails were more common. Also the proportion of shell colour varied between populations. Cain and Sheppard (1952) suggested that there were correlations between habitat and colour types and habitat and band types. Thus, the shell colour in a habitat tended to be similar to the colour of the habitat. These are the conditions required for both selection and drift.
In our sampling strategy we collected C. nemoralis shells (live or dead) in two horizontal transect areas, each of which was chosen for the diversity of vegetation. From these localities three samples were taken along each transect approximately 25m apart. This strategy relies on the use of repetition to increase the accuracy of our comparisons, as well as independence, which minimises the effects of gene flow - making our assumptions more precise. We have also controlled for height and the influence of other environments (excluding shrublands from our samples), which will enable us to compare them without extraneous variables. The snails collected were identified as being one of three main colour types: pink, brown, or yellow. The snails were also identified by a banding system that specified the number of bands, if any, were present. After samples were recorded the snails were discarded. By comparing the frequencies of the polymorphisms in each of the environments, we can seek to determine the relative importance of selection and genetic drift.
Our hypothesis suggests that natural selection is the key influence in the phenotypic variation seen within the species. If this is true we would expect to find a significant difference in phenotypic frequencies between the samples taken from the woodland and those taken from the grassland. Furthermore there would be no significant difference between the polymorphic frequencies of those samples taken from within the same environment. These differences would need to be consistent across the samples taken. If however our results do not reflect this, then we can conclude that genetic drift is the primary factor affecting the variation.
References Cain, Arthur J., and Philip M. Sheppard. (1952). The Effects of Natural Selection on Body Colour in the Land Snail Cepaea nemoralis. Heredity 6, 217-231.
Shannen Seldon*
April Li
Balnur Zhaisanbayeva
Shamoli Ahmed
Poppy Green
Cepaea nemoralis are considered to be highly polymorphic as they show obvious differences in both colour and number of bands. Polymorphism has long been of evolutionary interest, as directional selection in a population would seem to lead to the existence of the most advantageous type. C. nemoralis are also small and easy to collect, and due to their lack of mobility we can observe multiple generations at once, thus making it the ideal organism to study.
Biologists have found the population structure of snails quite conspicuous. They varied in size with little migration between populations, also amongst populations the distribution varied. In some populations banded snails dominated, whereas in others unbanded snails were more common. Also the proportion of shell colour varied between populations. Cain and Sheppard (1952) suggested that there were correlations between habitat and colour types and habitat and band types. Thus, the shell colour in a habitat tended to be similar to the colour of the habitat. These are the conditions required for both selection and drift.
In our sampling strategy we collected C. nemoralis shells (live or dead) in two horizontal transect areas, each of which was chosen for the diversity of vegetation. From these localities three samples were taken along each transect approximately 25m apart. This strategy relies on the use of repetition to increase the accuracy of our comparisons, as well as independence, which minimises the effects of gene flow - making our assumptions more precise. We have also controlled for height and the influence of other environments (excluding shrublands from our samples), which will enable us to compare them without extraneous variables.
The snails collected were identified as being one of three main colour types: pink, brown, or yellow. The snails were also identified by a banding system that specified the number of bands, if any, were present. After samples were recorded the snails were discarded. By comparing the frequencies of the polymorphisms in each of the environments, we can seek to determine the relative importance of selection and genetic drift.
Our hypothesis suggests that natural selection is the key influence in the phenotypic variation seen within the species. If this is true we would expect to find a significant difference in phenotypic frequencies between the samples taken from the woodland and those taken from the grassland. Furthermore there would be no significant difference between the polymorphic frequencies of those samples taken from within the same environment. These differences would need to be consistent across the samples taken. If however our results do not reflect this, then we can conclude that genetic drift is the primary factor affecting the variation.
References
Cain, Arthur J., and Philip M. Sheppard. (1952). The Effects of Natural Selection on Body Colour in the Land Snail Cepaea nemoralis. Heredity 6, 217-231.